The Gorgon’s Head!

by Nicholas Mee on July 11, 2017

A great cosmic drama plays out above our heads every night. Perseus, the Greek epic hero, was challenged to visit the land of the hyperboreans, a semi-mythical bleak and frosty land beyond the north wind known today as the island of Britain, his task to retrieve the head of the gorgon Medusa.

Medusa by Caravaggio

The beautiful Medusa had been seduced by the mighty sea god Poseidon and for this indiscretion she was severely punished. The goddess Athena transformed her glorious hair into a nest of serpents and proclaimed the curse that all who gazed on her face would be turned to stone. But Perseus was forewarned and he side-stepped the curse’s power. Viewing Medusa’s reflected image in his polished bronze shield, with his sword he severed her snake-wreathed head. At once her bleeding neck disgorged the offspring of her fatal encounter with the sea god, the warrior Chrysaor clad in shining armour and the wingéd horse Pegasus. Perseus took up the head of Medusa, mounted Pegasus and rose into the air on course for warmer climes.

On reaching the land of Egypt Perseus encountered a distressing scene. The princess Andromeda was chained to a rock about to be ravished by a terrible dragon. Andromeda’s parents King Cepheus and Queen Cassiopeia looked on helplessly, powerless to save the princess. Arriving just in time, Perseus slayed the dragon and saved Andromeda from her dreadful fate.

Perseus Freeing Andromeda by Piero di Cosimo. A painting with a wonderful air of Monty Python about it.

Six of the most northerly constellations in our night sky are intertwined with this mythic episode. Perseus, Andromeda, the square of Pegasus and the ‘W’ of Cassiopeia are very prominent features of the night sky. The stars of Cepheus are less obvious and so is the dragon Draco formed of a train of stars close to the north pole. These constellations were named at least two and a half thousand years ago.

Star map showing the northern constellations Cepheus, Draco, Perseus, Cassiopeia, Andromeda and Pegasus.

The Gorgon’s Head

Greek philosophers such as Aristotle drew a sharp distinction between the ideal realm of the heavens and the sublunar region of the Earth. The stars were believed to be constant and unchanging, shining steadily throughout eternity, whereas the Earth was subject to dissolution and decay. But these beliefs must have been questioned even in antiquity as not all stars are quite so constant and unchanging. In the constellation Perseus lies a curious star that represents the head of Medusa and this identification cannot be coincidental. The star is known as Algol and as with many prominent stars this name has an Arabic origin. Algol derives from Ra’s al Ghul, a term used by Islamic astronomers meaning ‘the head of the ghoul’. This in turn translates the expression used by Ancient Greek astronomer Ptolemy meaning ‘the gorgon’s head’. The remarkable feature of Algol that led to this identification is that every three days or so the star fades dramatically for around ten hours and then brightens again. The interval between these dips in brightness is precisely 2 days 20 hours and 49 minutes.

John Goodricke

The strange variability of Algol remained a mystery for many centuries. Not until 1782 did anyone offer an explanation. On 12 November 1782 the 18 year old John Goodricke was watching when Algol underwent its curious transformation. As he recorded in his journal, he was astonished by what he saw:

This night looked at Algol and was much amazed to find its brightness altered. It now appears to be fourth magnitude. I observed it diligently for about an hour upwards hardly believing that it changed its brightness, because I had never heard of any star varying so quick in its brightness. I thought it might be perhaps owing to an optical illusion, a defect in my eyes or bad air, but the sequel will show that its change is true and that it was not mistaken.

Goodricke was born in Gröningen in the Netherlands, the son of an English diplomat. At the age of five he contracted scarlet fever and was left profoundly deaf. Fortunately, his wealthy parents were able to send him to specialist schools in Edinburgh and Warrington where he received speech therapy and learnt to lip read. In his later school years he was introduced to astronomy and after leaving school he moved to York where he could observe the night sky with his cousin Edward Pigott in an observatory built by his uncle. It was there that he encountered the monstrous behaviour of Algol. But unlike earlier astronomers who witnessed the uncanny fading of Algol, Goodricke proposed an explanation.

John Goodricke painted in 1785 by James Scouler.

A Spectacular Discovery

William Herschel had made a spectacular discovery just the previous year, 1781 – the first new planet in recorded history, known today as Uranus. The existence of a new heavenly body orbiting the Sun in the far reaches of the solar system may have inspired Goodricke’s thoughts about Algol. Goodricke suggested that Algol might really be a pair of stars orbiting each other so closely that they could not be separated even with a telescope. He reasoned that if one of the stars was much brighter than the other, then each time the fainter star passed in front of the brighter star it would block some of its light and this partial eclipse would cause Algol to fade as viewed from Earth, and the regularity of the orbit would produce a clockwork predictability in the dimming events. Goodricke published his explanation the following year. It was immediately accepted and he was rewarded with the Copley Medal, the most prestigious award of the Royal Society.

Goodricke’s explanation is now known to be correct. Algol lies at a distance of 92.8 light years. It consists of a bright star of 3.17 solar masses and a second much fainter star of 0.7 solar masses separated by 9.3 million kilometres (0.062 astronomical units, where one astronomical unit is the distance between the Earth and the Sun). The two stars orbit each other in just under three days and the periodic eclipsing of the brighter star accounts for the periodic dimming of Algol just as Goodricke proposed. Remarkably there is also a third star of 1.76 solar masses in the system, which orbits the inner pair at a distance of around 400 million kilometres (2.69 a.u.) completing one orbit every 680 days, but this does not affect the observed brightest of the stars.

Goodricke continued to study variable stars and found other eclipsing binaries. He also discovered the variability of Delta Cephei, the fourth brightest star in the constellation Cepheus. This was the first discovery of a Cepheid variable, a class of stars that would play an essential role in determining the distances to other galaxies. Goodricke was the rising star of British astronomy. At the age of just 21 on 16 April 1786, he was elected Fellow of the Royal Society. Tragically, he never learnt of this great honour. He died of pneumonia just four days later.


Further Information

My book Gravity: Cracking the Cosmic Code contains lots more about the history of astronomy and the lives of the great astronomers:


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